Mobile soil optical mapping system

ABSTRACT

A soil mapping system for collecting and mapping soil reflectance data in a field includes an implement having a furrow opener for creating a furrow and an optical module. The optical module is arranged to collect soil reflectance data at a predetermined depth within the furrow as the implement traverses a field. The optical module includes two monochromatic light sources, a window arranged to press against the soil, and a photodiode for receiving light reflected back from the soil through the window. The two light sources have different wavelengths and are modulated at different frequencies. The photodiode provides a modulated voltage output signal that contains reflectance data from both of the light sources. Additional measurement devices are carried by the implement for collecting additional soil property data, such as electrical conductivity, pH, and elevation, which can be used together with the optical data to determine variations in soil organic matter.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/277,208 filed on Oct. 19, 2011, now U.S. Pat. No. 9,743,574, whichclaims priority of U.S. Provisional Application No. 61/394,740 filed onOct. 19, 2010. This application is a continuation-in-part of U.S.application Ser. No. 12/253,594 filed on Oct. 17, 2008, now U.S. Pat.No. 8,204,689, which claims priority of U.S. Provisional Application No.60/982,395 filed on Oct. 24, 2007. The entire contents of these priorapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to methods and devices foranalyzing and mapping soil properties within a field. In particular, thepresent invention relates to methods and devices for collecting andmapping soil reflectance data on-the-go.

Description of the Related Art

Variations in soil properties can be detected, even with the human eye,based on differences in light reflectance. Darker soils contain higherlevels of moisture or organic matter than light-colored soils. Moleculescontaining C—H, O—H, or N—H bonds that are exposed to light vibrate dueto the force of the electric field. This vibration absorbs opticalenergy so that less light is reflected off the soil. While this can bedetected visually, light sensors, especially those in the near infrared(NIR), can quantify the reflectance characteristics and provide the dataneeded to develop calibrations to soil properties. Soil reflectance hasbeen studied extensively since the 1970s and is widely reported in thescientific literature as an effective means for approximating soilorganic matter and carbon. There have been some uses of bare soilphotographs where the darker areas were correlated with higher organicmatter levels, but with the increased use of conservation tillage andno-till farming, the ability to collect such images has diminished.Rudimentary devices to collect reflectance data in the field, operatingnear or under the soil surface, were mobilized in the early 1990s. Dueto several limitations in their designs, neither of these was fullycommercialized.

Since the advent of GPS-enabled precision farming in the mid-1990s,growers have sought ways to better delineate areas of contrastingproductivity within their fields. Yield maps produced by combine yieldmonitors and remote crop imagery both show annual crop differences, butrelating those temporal variations to fundamental productivity zones hasproven challenging due to the many factors affecting crop growth. Soilsurveys produced by the USDA have also been examined, but the scale atwhich these were created is too coarse to show many important inclusionsof varying soils. On-the-go sensors to measure other soil propertieshave been developed and widely commercialized, including one thatmeasures soil pH, and several that relate soil electrical conductivitymeasurements to soil texture and soil salinity. These proximal sensorscollect dense datasets and their widespread use has generated anawareness of soil spatially variability within fields. None of thesecommercial sensors measures soil organic matter consistently.

Organic matter is an important factor in crop growth, as it affects soilmoisture infiltration and retention, soil tilth, rooting depth,soil-applied herbicide activity, nitrogen release, and other aspects ofnutrient cycling. A precise map of organic matter will provide growerswith an important piece of information as they seek to vary nitrogen,seed population, herbicides, and other inputs.

Veris Technologies Inc., the assignee of the present application, begandevelopment of soil optical devices in 2002 and has described acommercialized spectrophotometer system for mapping soil in its U.S.Patent Publication No. 2009-0112475 (Christy et al.). That systemincludes a field-deployed implement containing costly visible andnear-infrared spectrometers, which collect spectra that include over 300individual wavelengths. That level of technology is needed in soilresearch and where carbon measurements require an extremely high levelof precision, but is not practical for grower and consultant use due toexpense and complexity.

There is a need in the industry for a mapping system suitable for growerand consultant use, which is capable of providing accurate, useful soilorganic matter measurements using a simple, low cost design.

SUMMARY OF THE INVENTION

A soil mapping system for collecting and mapping soil reflectance datain a field includes an implement having a row unit with a furrow openerfor creating a furrow and an optical module. The optical module isarranged to collect soil reflectance data at a predetermined depthwithin the furrow as the implement traverses a field. An opening coulterand a pair of trash clearing disks can be used to clear residue and cuta slice in front of the furrow opener. The furrow opener in oneembodiment comprises two disks operating at a slight angle relative tothe direction of travel to form a V-shaped slot in the soil, similar todouble disk furrow opener used in a row unit of an agricultural planter.Gauge wheels can be positioned on each side of the furrow opener disksto maintain a consistent furrow depth and to scrape soil from the outersidewall of the disk during operation. The row unit is connected to atool bar by a parallel linkage, which allows the furrow opener andoptical module to follow ground undulations. An adjustable down-forcefeature allows the row unit to be adjusted to match soil conditions.Closing wheels or closing disks are provided to close the furrow behindthe optical module to prevent erosion.

The optical module includes two monochromatic light sources, a sapphirewindow arranged to press against the soil, and a single photodiode forreceiving light reflected back from the soil through the window. The twolight sources have different wavelengths and are modulated at differentfrequencies by a function generator contained in a controller. Thephotodiode provides a modulated voltage output signal that containsreflectance data from both of the light sources. The output signal fromthe photodiode is conditioned, converted to digital, and output asoptical data to a data logger or PC. Additional measurement devices arecarried by the implement for collecting additional soil property data,such as electrical conductivity, pH, and elevation, which can be usedtogether with the optical data to determine variations in soil organicmatter. A GPS signal is used to georeference all of the data.

According to one aspect of the present invention, a soil mapping systemis provided, comprising: an implement for traversing a field to bemapped: a furrow opener on the implement for creating a furrow as theimplement traverses a field; and an optical module on the implement. Theoptical module comprises at least one light source, a window arranged topress against soil at a predetermined depth within the furrow withconsistent pressure to provide a self-cleaning function, and aphotodiode for receiving light reflected back from the soil through thewindow.

According to another aspect of the present invention, a soil mappingsystem is provided, comprising: an implement for traversing a field tobe mapped; an optical module carried by the implement for collectingsoil reflectance data from soil in the field; and at least oneadditional measurement device carried by the implement for collectingdata for at least one soil property that relates to soil organic matter.The additional measurement device is selected from the group consistingof an electrical conductivity measurement device, an on-the-go soil pHmeasuring device, and an elevation measuring device. A means is alsoprovided for georeferencing data collected by the optical module and theadditional measurement device

According to another aspect of the present invention, an optical moduleis provided for a soil mapping system, the optical module comprising:two monochromatic light sources having different wavelengths which aremodulated at different frequencies; a window having an outside surfaceadapted to be pressed against soil; and a photodiode arranged to receivelight from the two light sources which is reflected back from the soilthrough the window. The photodiode having an output signal comprising amodulated voltage that contains soil reflectance data from both of thelight sources.

Numerous other objects of the present invention will be apparent tothose skilled in this art from the following description wherein thereis shown and described an embodiment of the present invention, simply byway of illustration of one of the modes best suited to carry out theinvention. As will be realized, the invention is capable of otherdifferent embodiments, and its several details are capable ofmodification in various obvious aspects without departing from theinvention. Accordingly, the drawings and description should be regardedas illustrative in nature and not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more clearly appreciated as thedisclosure of the present invention is made with reference to theaccompanying drawings. In the drawings:

FIG. 1 is an elevation view of a mobile soil mapping system according tothe present invention;

FIG. 2 is an elevation view of a row unit for an optical module of thesoil mapping system shown in FIG. 1;

FIG. 3 is an elevation view of a closing disk assembly used in the soilmapping system shown in FIG. 1;

FIG. 4 is an elevation view of a coulter assembly used for slicing soiland residue in front of the furrow opener assembly assembly for theoptical module;

FIG. 5 is a perspective view of the coulter assembly shown in FIG. 4;

FIG. 6 is a detail view of an optical module used in the soil mappingsystem;

FIG. 7 is a schematic view of the optical module and controller forcollecting reflectance data from the soil;

FIG. 8 is an elevation view of a mobile soil mapping system according toanother embodiment of the present invention;

FIG. 9 is an elevation view of a row unit for the optical module of thesoil mapping system shown in FIG. 8;

FIG. 10 is an elevation view of the row unit shown in FIG. 9, with theleft side gauge wheel and furrow opener disk removed to illustrate theoptical module;

FIG. 11 is a perspective view of the row unit shown in FIG. 9;

FIG. 12 is a perspective view of the row unit shown in FIG. 11, with theleft side gauge wheel and furrow opener disk removed to illustrate theoptical module;

FIG. 13 is a perspective view of the row unit for the soil mappingsystem shown in FIG. 8, together with the row unit mounting arrangementand controller.

FIG. 14 is a rear elevation view illustrating the optical modulepositioned in the V-shaped slot between the furrow opener disks.

FIG. 15 is a side elevation view illustrating the optical modulepositioned in the V-shaped slot between the furrow opener disks.

DETAILED DESCRIPTION OF THE INVENTION

A mobile soil mapping system for collecting on-the-go reflectancemeasurements of soil in a field according to the present invention willnow be described in detail with reference to FIGS. 1 to 13 of theaccompanying drawings.

The primary objective of the present invention is to collect on-the-gooptical measurements and correlate the data with soil organic matterlevels. The soil mapping system described herein minimizes interferencesfrom soil moisture and other sources of error through its mechanical,electronic, and data processing innovations.

Collecting high-quality optical measurements of soil in situ requirespreparing the soil scene so the sensor will have an ideal view of thesoil. This is accomplished in part by maintaining a consistent depth inthe soil. The consistent depth is important because simple opticaldevices have difficulty differentiating soil moisture from organicmatter, and soil moisture varies much more widely with depth than doesorganic matter. If the measurements are collected from a soil-engagingdevice that is bouncing over the field, the resulting data will beresponding to moisture variations much more than if the measurements areat a consistent depth.

It is also important that the measurement scene be free of dust, cropresidue, or mud that may adhere to the sensor. Therefore, themeasurement window on the optical module should be kept clean. If soilfrom another part of the field remains on the window, the system woulderroneously georeference the soil variations in the field.

FIGS. 1 to 5 illustrate an implement 10 having a frame with a toolbar 17supported above the soil by frame support wheels, and a speciallyconfigured row unit 11 used to collect optical measurements of soilaccording to the present invention. The row unit 11 includes a pair oftrash clearing wheels 12 for removing residue, a coulter 13 for cuttingthrough any remaining residue and for opening a slot in the soil, afurrow opener assembly 14 that creates a furrow in the soil, and anoptical module 15 having a window 16 arranged to be pressed against thesoil at a predetermined depth within the furrow. The row unit 11 can bemounted to the toolbar 17 of the implement 10 by a parallel linkage 18that allows the furrow opener 14 and optical module 15 (and the subframe22 on which they are mounted) to move vertically relative to the toolbar17 of the frame to follow ground undulations while maintaining aconsistent depth in the soil. A plurality of springs 19 or a pneumaticsystem (not shown) can be used to provide an adjustable down-force tomatch soil conditions.

The furrow opener 14 in the illustrated embodiment includes two disks 20that penetrate and follow in the slot created by the leading coulter 13.The disks 20 are arranged at a slight angle relative to a direction oftravel so as to form a V-shaped slot 50 or furrow in the soil. Forexample, the furrow opener 14 can be constructed in the same manner as aconventional double disk furrow opener used in an agricultural planter.Other types of furrow openers may also be used with the presentinvention.

The optical module 15 is mounted between the two furrow opener disks 20and is kept at a constant depth in the soil by being pressed against thebottom of the furrow while measurements are being made. The consistentpressure of the optical module 15 against the soil provides a selfcleaning function that prevents a buildup of soil on the window 16 ofthe optical module 15.

A pair of gauge wheels 21 are mounted in close proximity to the furrowopener disks 20 to control the operating depth of the disks 20 and toscrape off any soil that adheres to the outer surfaces of the disks 20during operation. The gauge wheels 21 are mounted together with thefurrow opener disks 20 and the optical module 15 on a subframe 22 of therow unit 11. The gauge wheels 21 maintain a consistent depth ofoperation of the optical module 15 in the soil during operation. Forexample, the gauge wheels 21 can be adjusted relative to the furrowopener disks 20 and optical module 15 to allow measurements to be takenat selected depths of approximately 1 to 3 inches below the soilsurface.

A furrow closing assembly 23 follows along behind the optical module 15to close the furrow after optical measurements are taken to preventerosion. The furrow closing assembly 23 can be a pair of closing disks24 as shown in FIGS. 1 and 3, or a pair of closing wheels 25 as shown inFIGS. 8 to 13.

The optical module 11 includes a single photodiode 30, a borosilicatephotodiode protection window 31, two different wavelengths of modulatingmonochromatic light sources 32, 33 modulated at different frequencies, atemperature sensor 34, and a wear plate 35 containing the sapphirewindow 16 that presses against the soil within the furrow. The modulatedlight is directed from the two light sources 32, 33 through the sapphirewindow 16 onto the soil. The reflected light is then received by thephotodiode 30, converted to a modulated voltage, and sent to acontroller 36. The photodiode 30 is hermetically sealed with theborosilicate window 31 protecting the surface. This allows for easycleaning, and is robust for outdoor use.

The controller 36 includes two function generators 37 for generating themodulated light from the two light sources 32, 33, a signal conditioningcircuit 38 including a phase lock loop (PLL) to separate each source ofreflected light from the photodiode signal, an analog to digital (A/D)converter 39, and a serial output 40 for data logging.

The function generators 37 send two separate pulses; one goes to thefirst wavelength light-emitting diode (LED) 32, the other to the secondwavelength LED 33. These pulses are directed at the soil through thesapphire window 16. The light reflected off the soil is read by thephotodiode 30 and converted into a modulated voltage. The modulatedvoltage from the single photodiode 30 is processed through the signalconditioning circuit 38, which converts the modulated voltage to a DCvoltage. The DC voltage is processed through the A/D converter 39, thenthe output is sent through the serial output 40 to the DataLogger or PC41. The data is georeferenced using a GPS signal from a GPS receiver 42connected to the DataLogger or PC 41.

By modulating the LEDs 32, 33 at two separate known frequencies andsending the modulated photodiode voltage to the PLL 38, each LED signalcan be extracted individually from the photodiode signal, withoutreceiving interference from the other LED light source or ambient light.This allows for a clean signal of only the reflected light of each LEDto be stored, free from any outside interference.

Correlating sensor data to soil properties requires the development ofcalibration equations. Previous calibration attempts with simple opticaldevices have relied on bivariate regression, with the optical data asthe sole sensor variable. One of the situations that can confoundoptical measurements of organic matter is soil moisture that relates tosoil texture variations in addition to relating to organic mattervariations.

The present invention includes the use of soil electrical conductivitysensors 43 for collecting electrical conductivity (EC) data in closeproximity to the optical module 15. The electrical conductivity sensors43 include rolling coulters that penetrate the soil and measure the soilEC at a given depth as the implement travels across the field. Soil EChas been proven to correlate well with soil texture. The presentinvention uses multivariate regression with EC and optical data to helpresolve the organic matter variations in the field.

The multivariate analysis is not limited to EC. The present inventionalso includes the use of an on-the-go soil pH sensor 44 that collectssoil pH data as the implement travels across the field, and a GPSreceiver 45 that provides elevation signals. Topography derivatives,such as slope, curvature and aspect, contribute to soil moisturevariations and can be derived from the elevation signals.

The dual wavelength optical module 15 of the present invention measureshow much light is reflected from the soil contacted by the window 16.Darker soils typically have higher organic matter levels, and a simpleregression model using lab-analyzed samples with the optical dataprovides reasonable calibrations. The model may be improved withaddition of other sensor data, using multi-variate regressiontechniques. Organic matter levels vary within a field for a variety ofreasons: landscape position, soil textures, and soil pH are key factorsaffecting organic matter development. Organic matter is formed bydecaying plant material, hence areas that produce more biomass havehigher organic matter levels as a result. Topography, soil texture andpH are key factors that affect biomass production. For example, mostplants don't grow as well on severely sloping ground, or on tightclaypan soils, or acidic soils as they do on gentle slopes, loam soils,and balanced pH soils. Soil pH also affects organic matter developmentwith microbial activity—certain soil microbes involved in the breakdownof plant material are inactive when pH is either very high or very low.If the regression model includes topography components such aselevation, slope, and curvature, derived from GPS data, or LIDARsensors, the model can account for organic matter difference based onlandscape position. If the model has soil texture information, such asis available from soil EC sensors, organic matter differences based ontextural changes are accounted for. Likewise, if soil pH data such as isavailable from on-the-go sensing is included, the model can make use ofthat information. Soil organic matter is a biological property that isrelated to soil physical properties such as topography and soil texture,and to soil chemical properties such as pH. Additional biological,physical, and chemical property information from sensors and othersources can also be included in the regression models used in thepresent invention.

An example of a calibration procedure that can be used with the presentinvention will now be explained. A database of optical, physical,chemical and biological soil information is assembled with Latitude andLongitude. Each data layer (optical, physical, chemical, and biological)is regressed to the soil property target using a leave 1 out validation.The leave 1 out algorithm removes 1 point from the database and uses theremaining points to predict the point removed. The process is repeateduntil all data points have been predicted. This provides a rigorousmethod for determining the best unbiased calibration.

After calculating a bivariate regression using each individual datalayer, multivariate regression using every data layer combination isalso conducted. The results are reported in a table containing metricssuch as R-squared (co-efficient of determination) RMSE (root mean squareerror of the prediction) and RPD (ratio of prediction to deviation);with the best results reported at the top and the poorest at the bottom.The best calibration models are applied to the entire field measurementsto provide a prediction for the soil target property.

Unique Features

At least the following features are believed to be unique to the soilmapping system of the present invention:

-   -   1. Dual wavelength, economical system with window in firm        contact with soil;    -   2. Mounting on specially configured row unit provides depth        control and holds furrow in place during measurement;    -   3. Single photodiode detector receives both wavelength signals;    -   4. Used in conjunction with soil EC sensors, elevation sensor,        and pH sensor, to improve calibration to specific soil        properties;    -   5. Coulter ahead of unit prepares scene for investigation;    -   6. Wheels or disks behind system close furrow to prevent        erosion; and    -   7. Multivariate data analysis of optical, EC, elevation, and pH        data for soil property calibration.

The present invention provides several advantages over existing soilmapping systems. For example, the depth control and soil scene creationof the present invention are better than the on-the-go spectrophotometerdescribed in U.S. Pat. No. 6,608,672 (Shibusawa) or thespectrophotometer described in U.S. Patent Publication No. 2009-0112475(Christy et al.).

The cost and complexity of the dual wavelength system of the presentinvention are much less than on-the-go spectrophotometers described byShibusawa and Christy et al.

Pressing the window of the optical module of the present inventionagainst the soil provides an advantage over Shibusawa because it allowsthe window to be self-cleaning.

Devices described in U.S. Pat. No. 5,044,756 (Gaultney) and U.S. Pat.No. 5,038,040 (Funk) did not include use of a window; while the windowof the present invention prevents dust and residue from occluding thesoil scene. Moreover, Gaultney used only one wavelength; the presentinvention uses two wavelengths to improve calibration to soilproperties.

Various modifications of the mobile soil mapping system of the presentinvention can be made without departing from the scope of the invention.For example, a window can be mounted on the side of the furrow openerr,either at 90° or at the same angle as the furrow. For another example,soil property estimates based on previous soil calibrations can be madeon-the-go and displayed on a computer in real-time. For another example,the system can be made to be portable or hand-held.

Other modifications are also possible, including the following:mechanical resistance sensor(s) can be added to the row unit; a soiltemperature sensor can be added; a soil moisture sensor can be added;the optical housing can be configured to measure the soil profile; andmeasurements can be used to control application of seed, fertilizer, orother material in real-time.

While the invention has been described in connection with specificembodiments thereof, it is to be understood that this is by way ofillustration and not of limitation, and the scope of the appended claimsshould be construed as broadly as the prior art will permit.

What is claimed is:
 1. A soil mapping system, comprising: an implementfor traversing a field to be mapped, said implement comprising a framewith a toolbar, and a row unit pivotally mounted to said toolbar, saidrow unit comprising a subframe mounted to said toolbar by at least onepivotal linkage member; a furrow opener mounted on the subframe forcreating a furrow as the implement traverses the field; an opticalmodule mounted on the subframe, said optical module comprising at leastone light source, a window arranged to press against soil in situ at apredetermined depth within said furrow with consistent pressure toprovide a self-cleaning function, and an optical receiver for receivinglight reflected back from the soil through the window; and at least onedepth gauging wheel mounted on the subframe in close proximity to thefurrow opener to control the operating depth of the furrow opener andthe optical module while allowing said subframe to move verticallyrelative to said toolbar to follow around undulations, whereby aconsistent operating depth of the furrow opener and the optical modulein soil can be maintained while said subframe is allowed to movevertically relative to said toolbar; wherein said furrow openercomprises a pair of rotatable disks arranged on the implement to form aV-shaped slot in the soil; and wherein said optical module is positionedin the V-shaped slot between said disks.
 2. The soil mapping systemaccording to claim 1, wherein said optical module is arranged to presssaid window against soil at the bottom of said furrow.
 3. The soilmapping system according to claim 1, wherein said at least one depthgauging wheel comprises a pair of depth gauging side wheels mounted inclose proximity to the disks to control the operating depth of the disksand to scrape off soil adhered to an outer surface of the disks.
 4. Thesoil mapping system according to claim 1, further comprising a residueclearing device for removing residue from in front of the furrow opener.5. The soil mapping system according to claim 4, wherein said residueclearing device comprises a fluted coulter for cutting residue and foropening a slot in the soil in front of the furrow opener.
 6. The soilmapping system according to claim 5, wherein said residue clearingdevice further comprises a pair of trash clearing wheels that clearresidue in front of said coulter.
 7. The soil mapping system accordingto claim 1, wherein said optical module further comprises a temperaturesensor.
 8. The soil mapping system according to claim 1, wherein saidoptical module comprises a wear plate, and wherein said window is asapphire window contained in said wear plate.
 9. A soil mapping system,comprising: an implement for traversing a field to be mapped, saidimplement comprising a frame with a toolbar, and a row unit pivotallymounted to said toolbar, said row unit comprising a subframe mounted tosaid toolbar by at least one pivotal linkage member; a furrow openermounted on the subframe for creating a furrow as the implement traversesthe field; an optical module mounted on the subframe, said opticalmodule comprising at least one light source, a window arranged to pressagainst soil in situ at a predetermined depth within said furrow withconsistent pressure to provide a self-cleaning function, and an opticalreceiver for receiving light reflected back from the soil through thewindow; and at least one depth gauging wheel mounted on the subframe inclose proximity to the furrow opener to control the operating depth ofthe furrow opener and the optical module while allowing said subframe tomove vertically relative to said toolbar to follow around undulations,whereby a consistent operating depth of the furrow opener and theoptical module in soil can be maintained while said subframe is allowedto move vertically relative to said toolbar; wherein said at least onepivotal linkage member comprises a parallel linkage that allows the rowunit to move vertically relative to said frame to follow groundundulations.
 10. The soil mapping system according to claim 9, whereinsaid furrow opener comprises a pair of rotatable disks arranged on theimplement to form a V-shaped slot in the soil.
 11. The soil mappingsystem according to claim 9, further comprising an adjustable down-forcemechanism associated with said parallel linkage for adjusting adown-force of the row unit to match soil conditions.
 12. The soilmapping system according to claim 9, wherein said row unit furthercomprises a pair of closing wheels or closing disks following theoptical module for closing the slot in the soil to prevent erosion. 13.The soil mapping system according to claim 9, wherein said at least onelight source of said optical module comprises two monochromatic lightsources having different wavelengths which are modulated at differentfrequencies.
 14. The soil mapping system according to claim 13, whereinsaid optical receiver comprises a single photodiode arranged to receivelight reflected back from the soil from each of said two monochromaticlight sources.
 15. A soil mapping system, comprising: an implement fortraversing a field to be mapped, said implement comprising a frame witha toolbar, and a row unit pivotally mounted to said toolbar, said rowunit comprising a subframe mounted to said toolbar by at least onepivotal linkage member; a furrow opener mounted on the subframe forcreating a furrow as the implement traverses the field; an opticalmodule mounted on the subframe, said optical module comprising at leastone light source, a window arranged to press against soil in situ at apredetermined depth within said furrow with consistent pressure toprovide a self-cleaning function, and an optical receiver for receivinglight reflected back from the soil through the window; and at least onedepth gauging wheel mounted on the subframe in close proximity to thefurrow opener to control the operating depth of the furrow opener andthe optical module while allowing said subframe to move verticallyrelative to said toolbar to follow ground undulations, whereby aconsistent operating depth of the furrow opener and the optical modulein soil can be maintained while said subframe is allowed to movevertically relative to said toolbar; wherein said at least one lightsource of said optical module comprises two monochromatic light sourceshaving different wavelengths which are modulated at differentfrequencies; wherein said optical receiver comprises a single photodiodearranged to receive light reflected back from the soil from each of saidtwo monochromatic light sources; and further comprising a controller forreceiving a modulated voltage from the photodiode, said controllercomprising two function generators for sending two separate pulses tothe two monochromatic light sources, a phase lock loop to separate eachsource of reflected light from the photodiode signal to form an output,an analog to digital converter to process the output, and a serialoutput, wherein the processed output is sent through the serial outputfor data logging.
 16. A soil mapping system, comprising: an implementfor traversing a field to be mapped, said implement comprising a framewith a toolbar, and a row unit pivotally mounted to said toolbar, saidrow unit comprising a subframe mounted to said toolbar by a parallellinkage that allows the row unit to move vertically relative to thetoolbar to follow ground undulations; a furrow opener mounted on thesubframe for creating a furrow as the implement traverses the field; anoptical module mounted on the subframe, said optical module comprisingat least one light source, a window arranged to press against soil insitu at a predetermined depth within said furrow with consistentpressure to provide a self-cleaning function, and an optical receiverfor receiving light reflected back from the soil through the window; andat least one depth gauging wheel mounted on the subframe in closeproximity to the furrow opener to control the operating depth of thefurrow opener and the optical module, whereby a consistent operatingdepth of the furrow opener and the optical module in soil can bemaintained while said subframe is allowed to move vertically relative tosaid toolbar via said parallel linkage.
 17. The soil mapping systemaccording to claim 16, wherein said furrow opener comprises a furrowopener disk, and said at least one depth gauging wheel is arranged toscrape soil from an outer sidewall of said furrow opener disk.
 18. Thesoil mapping system according to claim 16, wherein said furrow openercomprises a pair of furrow opener disks arranged at a slight anglerelative to a direction of travel so as to form a V-shaped furrow in thesoil, said optical module located between said furrow opener disks, saidat least one depth gauging wheel comprises a pair of gauge wheelsmounted on the subframe in close proximity to the furrow opener, andsaid gauge wheels are arranged to scrape soil from outer sidewalls ofsaid furrow opener disks.